The invention relates to a method and arrangement according to the preambles of the independent claims for measuring commutation delay of a frequency converter.
Methods based on frequency conversion for controlling electronic machines usually aim at making the output voltage of the frequency converter correspond to a specific reference value set for torque, flux, frequency, or other the like. The preset voltage value may therefore originate from a current or scalar controller, for example, in which case the lowest control loop is always a voltage controller. Although an actual voltage controller is not always needed, for example in methods based on direct torque control, also in these cases the realized value of average voltage should be known as precisely as possible to allow the machine to be controlled in an optimal manner. Consequently, good performance in motor control usually requires proper voltage feedback coupling.
An instantaneous output voltage of a frequency converter is usually derived on the basis of switch position and a measured intermediate circuit voltage. However, the magnitude of an average output voltage depends on the real lengths of phase-specific voltage pulses, which may vary depending on the duration of the dead time. Pulse length must therefore be measured to eliminate the impact of dead time. The measurement is usually carried out using a comparator circuit that compares the phase voltage with a half of a DC voltage, for example. The calculation provides the correct average voltage value provided that the real voltage changes linearly (or extremely rapidly) during the rising and falling edge. This situation is illustrated in FIG. 1, in which the area of the real voltage pulse corresponds to the signal generated by a comparator (broken line).
In practice voltage change is not always linear during changes in switch position. This problem arises particularly when commuting phase current is close to zero, in which case voltage change may be extremely vague. Comparison of output voltage on the basis of a half of a DC voltage may therefore lead to a considerable error in the interpretation of the pulse length. For example in the case illustrated in FIG. 2 the area of the real voltage pulse is clearly smaller than what could be expected on the basis of the signal (broken line) generated by the comparator.
In other words, comparison of instantaneous phase voltage is not always reliable for making conclusions on the magnitude of average output voltage. In practice this can be detected as a sixth harmonic wave in torque when driving at a low speed (<30 rpm), in which the shaft movement is often jerky so that it is visible to the eye if the speed controller has not been tuned tight. Problem spots, i.e. sector changes in which the current of some phase drops to zero for a moment, are clearly visible on the shaft.
The problem associated with phase voltage comparison is naturally aggravated when switching frequency is increased. This means that if voltage feedback is based on the above-disclosed conventional method of comparison, a lower performance of motor control must be accepted at higher switching frequencies (>8 kHz).
This problem in comparison could be avoided by measuring average phase voltage by means of analog integrators and AD converters, for example. However, in this solution problems arise from the price and the offset and gain errors of the analog components.
A number of methods have been proposed to resolve or to reduce the effects the above problem known from before. U.S. Pat. No. 5,206,802, for example, discloses a method and device for dead-time compensation in switch components. According to this method, a separate voltage required for the compensation is calculated and added to a voltage reference.
U.S. Pat. No. 4,547,719, in turn, discloses the measuring of output voltages of an inverter, these voltages being used for creating a feedback signal to compensate for dead time.
A problem with the prior art solutions is their complexity and the costs increase arising from the measurements and the feedback couplings.